RNAi (RNA interference) is a phenomenon in which small double-stranded
RNA (referred to as small interference RNA or siRNA) knocks down the expression
of its corresponding gene. RNAi has been observed in plants, C. elegans and
Drosophila, for a number of years. It was only until recently that RNAi was
discovered to work in mammalian systems.

Small interference RNA (siRNA) is 19-22 nt double-stranded RNA. It works by
cleaving and destroying its cognate RNA. siRNA first assembles into RNA-induced
silencing complexes (RISCs), and it then activates the complex by unwinding
its RNA strands. The unwound RNA strands subsequently guide the complex to the
complementary RNA molecules, where the complex cleaves and destroys the cognate
RNA, thus resulting in the RNAi phenomenon.

RNAi has evolved into a powerful tool to study gene function. Here are some
of its applications:

A stable cell line with a specific gene knocked-out can be established. Its
phenotype can be studied.

A knock-out mouse line can be established using transgenic siRNA method.

siRNA can be put into a vector with an inducible promoter to study its effect.

siRNA can be delivered by using viral vector and used for gene therapy purpose.

siRNA can be mimicked by chemical molecule and used for drug development.

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